How can social norms be changed?

Here’s one way to look at it. In a well-known (at least within the field of political science) article titled International norm dynamics and political change (1998), Finnemore & Sikkink suggest a way of thinking about this by presenting the norm life cycle. The life cycle has three distinct stages.

The first and most salient stage to your question is norm emergence. In this phase, a new (or emergent) norm is suggested by an individual or organisation referred to as norm entrepreneurs. Norm entrepreneurs typically have strong convictions or other interests that inspire them to want to shift the rules of appropriate behaviour in society. An effective norm entrepreneur calls attention to specific issues, and highlights current normative inconsistencies present within the status quo. By framing the issue in a certain narrative, they argue how the norm they are suggesting is, in fact, more appropriate. Often, frames are supported by referring to commonly held values and subsequently alluding to how these values may be threatened, or how behaviour could be improved in order to get closer to the ideal set out by said values. For example, if we everyone agrees that all men are created equal, how can we still condone slavery?

If the emergent norm gains traction and gets support by enough actors, the next stages of norm cascade and, finally, norm internalisation follow. Once internalised, the new norm has become embedded in what is considered appropriate behaviour within that social context, and transgressing it could result in consequences, ranging from raised eyebrows to punishment by death.

One way would be to change the individual’s perception of a descriptive norm (the existence or prevalence of). That is, convince an individual that relevant peers engage in a certain behavior, hold a certain belief, etc.

That is how a lot of persuasion work based in norms operates (e.g., Theory of Normative Behavior).